Discuss Keats's and Robert Browning's use of imagery in their poetry. I am interested in Browning's "Porphyria's Lover" and "My Last Duchess."

I'll answer concerning the Browning poems you are
interested in, and let another editor compare them to Keats'
imagery.

In "Porphyria's Lover," Browning is addressing the
obsession the outwardly polite and prudish Victorian society had with sensational
stories of horror and depravity.  The themes of sex and violence and madness in this
dramatic monologue speak to this obsession.

Browning turns
the conventional presentation of these issues and themes, however, by making them seem
natural and beautiful.  Porphyria glides (line 6) in amidst the rain and the wind and
shuts out the cold and the storm (line 7).  She builds a fire that warms the cottage
(line 9).  She bares her shoulder (line 17) and lays her hair upon his cheek (line 19). 
These are all images that create beauty and comfort.

At a
poignant (a feeling of specialness) moment, she totally gives herself to her lover, and
he, trying to preserve the moment, strangles her with her own hair, painlessly,
according to the speaker. 

Browning forces readers to
contemplate the relationship between sex and violence and power and complex madness, as
well as beauty. 

Browning uses imagery to a different
effect in "My Last Duchess."  The portrait of his murdered wife may be beautiful, as may
his other works of art, but beauty is not the issue, and isn't what's meaningful to the
Duke.  His wife is now, in the present of the poem, in a perfect state.  She is the
perfect work of art, reflecting back on him.  He kills her because she does not behave
as a work of art.  As a portrait, she does.  The Duke is interested in how works or art
reflect back on him and his 900-year-old name. 

One of the
key images in this dramatic monologue is his wife's countenance, her face, its depth and
passion revealed in her glance as captured in the portrait (lines 7-21).  But the image
isn't important for its beauty, but for how it's interpreted and twisted by the Duke. 
Almost anything could have brought that "spot of joy" to her face, says the Duke.  His
wife had the audacity to be polite when anyone showed her "courtesy," according to the
Duke.  She was "too soon made glad" (line 22). 

The images
that follow, the daylight, cherries, the white mule, all reveal that which might make
his wife smile.  And that is no good. 

The Duke is much
happier now that she is truly a work of art and, therefore, behaves like one.  Now she
smiles only when he pulls back the curtain and allows someone to look at her--someone
like his silent listener.